1. Field of the Invention
The present invention relates to an electronic musical instrument adapted for digitally changing a musical waveform with time and in accordance with a touch response through utilization of a switched capacitor filter circuit for controlling a harmonic component of a source waveform signal in accordance with a desired timbre.
2. Description of the Prior Art
Conventionally, a musical tone generation system for electronic musical instruments is roughly divided into an analog sound source system and a digital sound source system, which respectively have merits and demerits. The digital sound source system permits synthesization of a musical waveform element by digital calculations, and hence is capable of producing timbre over a wide range, but since it posseses the defects of an enormous circuit scale and limitations imposed on the synthesization of timbre by the amount of calculation and the time therefor, it is employed only in some high-grade models. On the other hand, the analog sound source system comprises a source waveform generator for generating a source waveform signal corresponding to a note frequency and containing harmonic components in abundance, a filter circuit for controlling the harmonic components of the source waveform signal in accordance with a timbre desired to produce and an envelope generator for generating a desired envelope. These circuits have undergone various improvements as well-known VCO (Voltage Controlled Oscillator), VCF (Voltage Controlled Filter) and VCA (Voltage Controlled Amplifier) which employ voltage as a common control parameter. Further, many proposals have been made on digital control of electronic musical instruments through utilization of digital electronic circuit technologies typified by the microcomputer technology. For example, in the source waveform generator, the pitch corresponding to a note frequency is obtained with high accuracy by DCO (Digital Control Oscillator) using a "program counter". Moreover, storage and display of timbre setting data on the control panel, instantaneous modification of may musical parameters and simultaneous control of a plurality of channels by time-shared operations can also e achieved easily by the employment of the microcomputer technology.
However, many problems have been pointed out in connection with the digitization of an electronic musical instrument of the conventional analog sound source system. For instance, the VCF (Voltage Controlled Filter) could have been constructed at a low cost since it is necessary only to supply voltage of a volume specified on the control panel, but the supply of data via the microcomputer involves an A/D conversion and a D/A conversion, and hence calls for a very complex circuit arrangement. Further, in order to digitize control of an EG (Envelope Generator) for generating a control voltage which is supplied to the VCF to obtain temporal variations of timbre, it is necessary to supply the voltage after D/A converting it for each of such parameters as A (Attack), D (Decay), S (Sustain) and R (Release). This requires a large-scale circuit arrangement and analog fine control (offset control). Besides, in view of the characters of circuit parts forming the VCF, it is very difficult to employ an analog LSI in which a large-scale circuit is formed on one chip, and the manufacturing costs also present a problem.